Method and device for handling base sequences in a communications network

ABSTRACT

The embodiments herein related to a method in a network node, a network node, a method in a user equipment and a user equipment for handling base sequences in a communications network. The network node is configured to communicate with a first user equipment. The network node comprises information about a default base sequence and an alternative base sequence. The network node determines, for the first user equipment, that the alternative base sequence should replace the default base sequence. The network node sends information about the determined alternative base sequence to the first user equipment.

TECHNICAL FIELD

Embodiments herein relate generally to a network node and a method inthe network node, and to a user equipment and a method in the userequipment. More particularly the embodiments herein relate to handlingbase sequences in a communications network.

BACKGROUND

In a typical cellular network, also referred to as a communicationsystem or wireless communication system, User Equipments (UEs),communicate via a Radio Access Network (RAN) to one or more corenetworks (CNs).

A user equipment is a mobile terminal by which a subscriber may accessservices offered by an operator's core network and services outsideoperator's network to which the operator's RAN and CN provide access.The user equipments may be for example communication devices such asmobile telephones, cellular telephones, or laptops with wirelesscapability. The user equipments may be portable, pocket-storable,hand-held, computer-comprised, or vehicle-mounted mobile devices,enabled to communicate voice and/or data, via the radio access network,with another entity, such as another mobile station or a server.

User equipments are enabled to communicate wired or wirelessly in thecommunications network. The communication may be performed e.g. betweentwo user equipments, between a user equipment and a regular telephoneand/or between the user equipment and a server via the radio accessnetwork and possibly one or more core networks, comprised within thecellular network.

The communications network covers a geographical area which is dividedinto cell areas. Each cell area is served by a Base Station (BS), e.g. aRadio Base Station (RBS), which sometimes may be referred to as e.g.“eNB”, “eNodeB”, “NodeB”, “B node”, or Base Transceiver Station (BTS),depending on the technology and terminology used. A “cell” ischaracterized in e.g. Long Term Evolution (LTE) by a “cell-ID”, whichaffects several cell-specific algorithms and procedures.

The base stations may be of different classes such as e.g. macro eNodeB,home eNodeB or pico base station, based on transmission power andthereby also on cell size.

The base station communicates, over radio carriers or channels, with oneor more user equipment(s) using a radio access technology, such as e.g.LTE, LTE Advanced, Wideband Code Division Multiple Access (WCDMA),Global System for Mobile Communications (GSM), or any other ThirdGeneration Partnership Project (3GPP) radio access technology. LTE isused as an example in the following description.

When the base station receives, at its antenna(s), signals from aplurality of user equipments, it may use different reception techniquesfor demodulation. Two different reception techniques for demodulatingthe symbols from multiple user equipments in each cell are SuccessiveInterference Cancellation (SIC) and Interference Rejection Combining(IRC). Both of these reception techniques require a baseband receiver,at the base station, to estimate the channel between each user equipmentand each base station antenna. Baseband refers to signals and systemswhose range of frequencies is measured from close to 0 hertz to acut-off frequency, a maximum bandwidth or highest signal frequency.Baseband may also be used as a noun for a band of frequencies startingclose to zero. The quality of the channel estimates greatly influencesthe performance of both SIC and IRC.

The base station may comprise multiple antennas, and the base stationmay receive signals from a user terminal at the multiple antennas. Toreceive a signal from a specific user equipment, the base stationdetermines the set of base station antennas that will be used to receivethe signal transmitted from the user equipment. The signals received bythis set of antennas are sent to an “uplink receiver” that demodulatesthe signal transmitted by the user equipment. Note that the same set ofantennas could be used for the reception of multiple user equipments.The uplink receiver typically estimates the uplink channels between eachuser equipment and base station antenna using reference signals that aretransmitted from each user equipment on the uplink. When the basestation estimates the uplink channel from a particular user equipment,the reference signals from other user equipments in the network act asinterference and degrade the accuracy of the channel estimation.Therefore, it is generally desirable that the reference signals from allthe user equipments are mutually orthogonal. In an LTE system, given onereference signal spanning consecutive subcarriers, a second orthogonalreference signal spanning the same subcarriers may be generated byadding a linear phase rotation to the same base reference signal. Byusing different phase rotations for different user equipments, a largenumber of mutually orthogonal reference signals spanning the samesubcarriers may be generated.

The communication between a base station and a user equipment may bestructured in different ways, depending on the technology which is used.For example, in LTE, the communication is structured in frames andsubframes. One type of LTE frame, i.e. Time Division Duplex (TDD) mode,has an overall length of 10 ms. The frame is divided into 20 individualslots. A subframe comprises two slots, i.e. there are ten subfameswithin a frame. Another type of an LTE frame, i.e. Frequency DivisionDuplex (FDD) mode, comprises two half frames, each having an overalllength of 5 ms. And each half frame is split into five subframes, each 1ms long.

An LTE communications network is designed to support user equipmentsfrom different releases, i.e., Rel-8/9/10/11, in a backward compatibleway. One of the LTE network design objective is to enable co-schedulingof such user equipments in time, frequency and space, i.e. Multi-UserMultiple Input Multiple Output (MU-MIMO), dimensions with as fewscheduling constraints as possible.

Furthermore, the LTE standard should be able to support various andflexible deployments. Some examples of expected deployments for modernLTE networks, i.e. Rel-11 and beyond, comprise, e.g.,

-   -   Macro-deployments, where large cells are typically divided into        independent sectors.    -   Hetrogenous Networks (HetNet)-deployments, where pico-cells are        deployed within the coverage of macro-cell in order, e.g., to        improve coverage for high data rate user equipments.    -   Hotspot scenarios where an access point serves a small area with        high throughput need.

In addition, LTE networks are designed with the aim of enabling optionalCoordinated Multipoint Processing (CoMP) techniques, where differentsectors and/or cells operate in a coordinated way in terms of, e.g.,scheduling and/or processing. An example is uplink CoMP where the signaloriginating from a single user equipment is typically received atmultiple reception points and jointly processed in order to improve thelink quality. Uplink joint processing, also referred to as uplink CoMP,allows transformation of what is regarded as inter-cell interference ina traditional deployment into a useful signal. Therefore, LTE networkstaking advantage of uplink CoMP may be deployed with smaller cell sizecompared to traditional deployments in order to fully take advantage ofthe CoMP gains.

The uplink of LTE is designed assuming coherent processing, i.e., thereceiver is able to estimate the radio channel from a transmitting userequipment and to take advantage of such information in a detectionphase, i.e. in demodulation of a received signal. Therefore, eachtransmitting user sends a Reference Signal (RS) associated to eachuplink data channel, i.e. Physical Uplink Shared Channel (PUSCH). Thereference signal may also be called pilot signal and are inserted in thetransmitted signal. The reference signals are sent fairly often as thechannel conditions change due to fast fading and other changes.

Each reference signal is characterized by a group-index and asequence-index. The reference signal is derived from a base sequence.Cyclic shift may be used for deriving the reference signal from the basesequence. In other words, multiple reference signal sequences aredefined from each base sequence.

Base sequences are cell-specific in Rel-8/9/10 and they are a functionof the cell-ID. Different base sequences are semi-orthogonal. Thereference signal for a given user equipment is only transmitted on thesame bandwidth of PUSCH, and the base sequence is correspondinglygenerated so that the reference signal is a function of the PUSCHbandwidth. For each subframe, two reference signals are transmitted, oneper slot.

There are two types of uplink reference signals: a demodulationreference signal (DMRS) and a Sounding Reference Signal (SRS). Thedemodulation reference signal is used for channel estimation for datademodulation, and the sounding reference signal is used for userscheduling.

Reference signals from different user equipments within the same cellpotentially interfere with each other and, assuming synchronizednetworks, even with reference signals originated by user equipments inneighboring cells. In order to limit the level of interference betweenreference signals different techniques have been introduced in differentLTE releases in order to allow orthogonal or semi-orthogonal referencesignals. The design principle of LTE assumes orthogonal referencesignals within each cell and semi-orthogonal reference signals amongdifferent cells, even though orthogonal reference signals may beachieved for aggregates of cells by so called “sequence planning”.

Orthogonal reference signals may be achieved by use of Cyclic Shift (CS)in Rel-8/9 or by CS in conjunction with Orthogonal Cover Codes (OCC) inRel-10. It is assumed that CS and OCC may also be supported by Rel-11user equipments.

Cyclic shift is a method to achieve orthogonality based on cyclic timeshifts, under certain propagation conditions, among reference signalsgenerated from the same base sequence. Only eight different cyclic shiftvalues may be indexed in Rel-8/9/10, even though in practice less thaneight orthogonal reference signals may be achieved depending on channelpropagation properties. Even though cyclic shift is effective inmultiplexing reference signals assigned to fully overlapping bandwidths,orthogonality is lost when the bandwidths differ and/or when theinterfering user equipments employ another base sequence.

OCC is a multiplexing technique based on orthogonal time domain codes,operating on the two reference signals provided for each uplinksubframe. The OCC code [1 −1] is able to suppress an interferingreference signal as long as its contribution after the base stationmatched filter is identical on both reference signals of the samesubframe. Similarly, the OCC code [1 1] is able to suppress aninterfering reference signal as long as its contribution after the basestation matched filter has an opposite sign respectively on the tworeference signals of the same subframe. The matched filter will bedescribed in more detail below.

While base sequences are assigned in a semi-static fashion, CS and OCCare user equipment specific and dynamically assigned as part of thescheduling grant for each uplink PUSCH transmission.

Even though joint processing techniques may be applied for PUSCH,channel estimates based on reference signals are typically performed inan independent fashion at each reception point, even in case of uplinkCoMP. Therefore, it is crucial to keep the interference level at anacceptably low level, especially for the reference signals.

In order to minimize the impact of burst interference peaks on referencesignals, interference randomization techniques have been introduced inLTE. In particular:

-   -   Cyclic shift randomization is always enabled and generates        random cell-specific cyclic shift offsets per slot. The        pseudo-random CS pattern is a function of the base sequence        index and the cell-ID and is thus cell-specific.    -   Sequence hopping and Group Hopping (SGH) are base sequence index        randomization techniques which operate on a slot level with a        cell-specific pattern, which is a function of the cell-ID and        sequence index.        -   For Rel-8/9 user equipments, SGH may be enabled/disabled on            a cell-basis.        -   For Rel-10 user equipments, SGH may be enabled in a user            equipment specific fashion.

In the uplink for LTE Rel-10 multi-antenna techniques which maysignificantly increase the data rates and reliability of a communicationsystem is introduced. The performance is in particular improved if boththe transmitter and the receiver are equipped with multiple antennas.This result in a MIMO communication channel and such systems and/orrelated techniques are referred to as MIMO.

LTE Rel.10 supports a spatial multiplexing mode, i.e. Single User-MIMO(SU-MIMO), in the communication from a single user equipment to the basestation. SU-MIMO is aimed for high data rates in favorable channelconditions. SU-MIMO comprises the simultaneous transmission of multipledata streams on the same bandwidth, where each data stream is usuallytermed as a layer. Multi-antenna techniques such as linear precoding areemployed at the transmitter in order to differentiate the layers in thespatial domain and allow the recovering of the transmitted data at thereceiver side.

Another MIMO technique supported by LTE Rel.10 is MU-MIMO, wheremultiple UEs belonging to the same cell are completely or partlyco-scheduled on the same bandwidth and time slots. Each UE in theMU-MIMO configuration may possibly transmit multiple layers, thusoperating in SU-MIMO mode.

In case of SU-MIMO it is necessary to allow the receiver to estimate theequivalent channel associated to each transmitted layer of each userequipment in order to allow detection of all the data streams. In caseof CoMP, such requirement applies also to user equipments belonging toother cells but comprised in the joint processing cluster. Therefore,each user equipment need to transmit a unique reference signal at leastfor each transmitted layer. The base station receiver is aware of whichreference signal is associated to each layer and performs estimation ofthe associated channel by performing a channel estimation algorithm. Theestimated channel is then employed by the receiver in the detectionprocess.

In case of MU-MIMO, user equipments may be scheduled on fully orpartially overlapping bandwidths. Some typical application cases areexemplified in the following:

-   -   MU-MIMO within a cell, fully overlapping bandwidth: in this case        the reference signals of the different user equipments may be        multiplexed by means of CS and/or OCC. Furthermore, SGH may be        enabled without affecting orthogonality.    -   MU-MIMO within a cell, partly overlapping bandwidth: in this        case the reference signals of the different user equipments may        be multiplexed by means of OCC only and SGH cannot be enabled        for any of the user equipments.    -   MU-MIMO of user equipments belonging to different cells, e.g.,        in a CoMP application, in this case the user equipments are        typically assigned different base sequences and orthogonality        may not be achieved, due to the different CS hopping patterns.

The deployments described above and the extensive use of uplink CoMPrequire scheduling flexibility and improved channel estimation quality,even for geographically far away user equipments belonging to anothercell. Assuming, e.g., a HetNet deployment, the small cell radius of thepicocell and the geographic location within the macrocell coverageimplies the presence of potentially strong interference between userequipments belonging to such cells. Densifications of the cells,increasing number of receive antennas and optional CoMP processing, onthe other hand, emphasizes the need for flexible MU-MIMO scheduling. Inthe scenarios described above, disabling SGH will enhance the risk ofinter-cell interference peaks.

SUMMARY

The presence of a mix of multiple user equipments from Rel-8/9/10 andbeyond in the same network emphasizes the need to seamlessly co-schedulesuch user equipments, independently of their specific release. However,MU-MIMO is not efficient in Rel-8/9/10 in conjunction with sequence andgroup hopping (SGH) if the paired user equipments are assigned differentbase sequences, because neither orthogonal cover codes (OCC) nor cyclicshifts (CS) are effective in such scenario and only semi-orthogonalitymay be achieved.

As an example, consider a case where two user equipments UE1 and UE2 areco-scheduled on the same bandwidth and consider that UE1 and UE2 belongto different cells and are not assigned the same base sequence. Anexample is that UE1 belongs to a macro-cell and UE2 to a pico-cell in ahetnet LTE scenario. Since the base sequences associated respectively toS1 and S2 are different, orthogonality between the UEs is not possiblewith consequent performance degradation due to cell-edge interference.

A solution may be to disable SGH in a user equipment specific way forsome of the Rel-10 user equipments. However, SGH may only be disabled ina cell-specific way for Rel-8/9 user equipment, implying cell-specificSGH disabling even for Rel-10 user equipments, with severe degradationof inter-cell interference.

Another solution may be to assign the same base sequence, andconsequently SGH pattern, to interfering cells such as, e.g., themacrocell and the picocells within the macrocell coverage. However,problems are associated with such a solution such as, e.g., reduced SGHrandomization, unpredictably large interference peaks generated whenuser equipments with the same base sequence are scheduled on partlyoverlapping bandwidths and DeModulation Reference Sequence (DMRS)capacity limitations, only CS and OCC may be employed fororthogonalizing DMRS over the aggregated cells.

An objective of embodiments herein is therefore to obviate at least oneof the above disadvantages and to provide improved channel estimation ina communications network.

According to a first aspect, the objective is achieved by a method in anetwork node for handling base sequences in a communications network.The network node is configured to communicate with a first userequipment. The network node comprises information about a default basesequence and an alternative base sequence. The network node determines,for the first user equipment, that the alternative base sequence shouldreplace the default base sequence. The network node sends informationabout the determined alternative base sequence to the first userequipment.

According to a second aspect, the objective is achieved by a method in afirst user equipment for handling base sequences in a communicationsnetwork. The first user equipment is configured to communicate with anetwork node. The first user equipment employs a default base sequence.The user equipment receives, from the network node, information that analternative base sequence should replace the default base sequence. Theuser equipment replaces the default sequence with the alternative basesequence.

According to a third aspect, the objective is achieved by a network nodefor handling base sequences in a communications network. The networknode is configured to communicate with a first user equipment. Thenetwork node comprises information about a default base sequence and analternative base sequence. The network node further comprising adetermining unit which is configured to determine, for the first userequipment, that the alternative base sequence should replace the defaultbase sequence. The network node comprises a sending unit configured tosend information about the determined alternative base sequence to thefirst user equipment.

According to a fourth aspect, the objective is achieved by a userequipment for handling base sequences in a communications network. Theuser equipment is configured to communicate with a network node. Theuser equipment employs a default base sequence. The user equipmentcomprises a receiving unit configured to receive, from the network node,information that an alternative base sequence should replace the defaultbase sequence. The user equipment further comprises a processing unitconfigured to replace the default sequence with the alternative basesequence.

Embodiments herein afford many advantages, of which a non-exhaustivelist of examples follows:

One or some embodiment(s) herein provide the advantage of providing amarginal complexity and allowing reuse of the sequence and group hopping(SGH) sequences implemented in Rel-8/9/10 UEs.

Furthermore, one or some embodiment(s) herein provide the advantage ofreducing interference of reference signals by enabling SGH for MU-MIMO.

Further, one or some embodiment(s) herein provide the advantage ofimproved scheduling flexibility for MU-MIMO.

At least one of the embodiments herein provide the advantage of allowingMU-MIMO between user equipments of different releases without disablingSGH, e.g. between Rel-11 LTE user equipments and user equipments fromprevious LTE releases.

An even further embodiment is that orthogonality between the userequipments is possible without consequent performance degradation due tocell-edge interference.

Some embodiments herein minimize signalling overhead and preserveflexibility in scheduling allocation.

The embodiments herein are not limited to the features and advantagesmentioned above. A person skilled in the art will recognize additionalfeatures and advantages upon reading the following detailed description.

Introduction of the alternative base sequence which may be dynamicallytriggered by scheduling grants enables achieving improved RSorthogonality of Rel-11 user equipments with either Rel-8/9/10/11 UE.Whenever a switch to an alternative base sequence is indicated, all thebase-sequence specific parameters, e.g., hopping offsets for SGH and CShopping, are correspondingly dynamically adjusted. By choosing thealternative sequence properly, e.g., for a CoMP setting, it is possibleto allow perfect or at least close to perfect RS orthogonality of Rel-11user equipments with Rel-8/9/10 and Rel-11 user equipment(s).Differently from prior art, orthogonality is thereby achieved both whenSGH is enabled and disabled for the Rel-8/9/10 user equipment.

The embodiments herein are not limited to the features and advantagesmentioned above. A person skilled in the art will recognize additionalfeatures and advantages upon reading the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments herein will now be further described in more detail inthe following detailed description by reference to the appended drawingsillustrating the embodiments and in which:

FIGS. 1 and 1 a each show a schematic block diagram illustratingembodiments of a communications network.

FIG. 2 is a schematic block diagram illustrating embodiments of aUL-DMRS subframe for UE1.

FIG. 3 is a schematic block diagram illustrating embodiments of aUL-DMRS subframe for UE2.

FIG. 4 is a combined signaling diagram and flow chart illustratingembodiments of a method.

FIG. 5 is a flow chart illustrating embodiments of a method in a networknode.

FIG. 6 is a schematic block diagram illustrating embodiments of anetwork node.

FIG. 7 is a flow chart illustrating embodiments of a method in a userequipment.

FIG. 8 is a schematic block diagram illustrating embodiments of a userequipment.

FIG. 9 is a flow chart illustrating embodiments of a method in a networknode.

FIG. 10 is a flow chart illustrating embodiments of a method in a userequipment.

The drawings are not necessarily to scale and the dimensions of certainfeatures may have been exaggerated for the sake of clarity. Emphasis isinstead placed upon illustrating the principle of the embodimentsherein.

DETAILED DESCRIPTION

FIG. 1 and FIG. 1a each depict a communications network 100 in whichembodiments herein relating to signaling of demodulation referencesignal (DMRS) patterns for multicell scenarios such as for a CoMPnetwork of cells, may be implemented. The communications network 100 mayin some embodiments apply to one or more radio access technologies suchas for example LTE, LTE Advanced, WCDMA, GSM, or any other 3GPP radioaccess technology.

The communications network 100 comprises network nodes such as e.g. abase station 103 serving a cell 101. The base station 103 may be a basestation such as a Radio Base Station, NodeB, an evolved NodeB (eNB),depending on the technology and terminology used, or any other networkunit capable to communicate over a radio carrier 102 with a first userequipment 105 being present in the cell 101. The radio carrier 102 mayalso be referred to as carrier, radio channel, channel, communicationlink, radio link or link. The base station 103 may be of differentclasses, for example a macro base station, such as for example a eNodeB,or a low power base station, such as for example a home eNodeB, picobase station, or femto base station, based on transmission power andthereby also on cell size. Even though FIGS. 1 and 1 a show the basestation 103 serving one cell 101, the base station 103 may serve two ormore cells 101. The communications network 100 may further comprise asecond user equipment 107 and a third user equipment 109. In someembodiments, the second user equipment 107 and the third user equipment109 are present in the same cell 101 as first user equipment 105 andserved by the same base station 103. In other embodiments, the first andthird user equipment 105, 109 are located in one cell and the seconduser equipment 107 is located in another cell but they may still belongto the same CoMP scheduling cluster, i.e. they are located in neighbourcells (FIG. 1a ).

The communications network 100 may be divided into cells, such as e.g.the cells 101. The use of cells is the reason why a communicationsnetwork 100 may be referred to as a cellular communications network. Acell is a geographical area where the base station 103 which serves thecell 101, provides radio coverage to user equipments 105 present in thecell 101. A cell 101 may be of different size such as e.g. a micro cellwhich typically covers a limited area, a pico cell which typicallycovers a small area, a femto cell which is typically designed for use ina home or small business or a macrocell which typically providescoverage larger than a microcell.

The user equipment 105 present within the cell 101 and served by thebase station 103 is in this case capable of communicating with the basestation 103 over the radio carrier 102. A data stream(s) is communicatedbetween the base station 103 and the user equipment(s) 105 over theradio channel 102 in a layered approach. Examples of layers are physicallayer, data link layer, network layer, transport layer, session layer,etc.

The user equipment 105 may be any suitable communication devices orcomputational devices with communication capabilities capable tocommunicate with the base station 103 over the radio channel 102, forinstance but not limited to mobile phone, smart phone, personal digitalassistant (PDA), laptop, MP3 player or portable DVD player (or similarmedia content devices), digital camera, or even stationary devices suchas a PC. A PC may also be connected via a mobile station as the endstation of the broadcasted/multicast media. The user equipment 105 maybe embedded communication devices in e.g. electronic photo frames,cardiac surveillance equipment, intrusion or other surveillanceequipment, weather data monitoring systems, vehicle, car or transportcommunication equipment, etc. The user equipment 105 is referred to asUE in some of the figures. Only one user equipment 105 is illustrated inFIG. 1 and FIG. 1a for the sake of simplicity; however the respectivebase station 103 may serve a set of plural user equipments 105.

It should be noted that the radio carrier 102 between the base station103 and the user equipment 105 may be of any suitable kind comprisingeither a wired or wireless link. The carrier 102 may use any suitableprotocol depending on type and level of layer, e.g. as indicated by theOpen Systems Interconnection (OSI) model, as understood by the personskilled in the art.

The following description uses an UpLink (UL) transmission path of anLTE Rel-11 network as an example, even though it may be applied even tothe Down Link (DL) and to other communication protocols, such as e.g.the ones mentioned above. The uplink (UL) is the link from the userequipment to the base station, and downlink (DL) is the link from thebase station to the user equipment.

Consider a subframe, S1, transmitted by a first user equipment UE1 basedon LTE Rel-8/9/10 and provided with two DMRS, respectively one per slot,as illustrated in FIG. 2. FIG. 2 represents a single transmission layerfor UL-DMRS subframe for the first user equipment, UE1. DMRS may also bereferred to as reference signals (RS). Without loss of generality, inthe following a time-domain representation of the signals is provided,but equivalent principles may be applied for frequency-domainprocessing. The x-axis of FIG. 2 illustrates time in, for examples,seconds. Let s₁ be the DMRS base sequence for slot-1 and s₂ the DMRSbase sequence for slot-2. In the case of multi-antenna transmission.

Consider now a second LTE subframe, S2, as illustrated in FIG. 3, wherethe DMRS base sequences for the two slots are respectively s3 and s4.The second LTE subframe, S2, is transmitted by a second LTE userequipment, UE2. FIG. 3 illustrating the uplink DMRS subframe for thesecond user equipment UE2 has an x-axis representing time, in forexamples seconds.

Assuming that SGH is enabled, subframes S1 and S2 having differentbase-sequences on both slots, where s1, s2, s3 and s4 aresemi-orthogonal base-sequences pseudo-randomly chosen from a set ofpredefined base sequences.

Then consider a case where the two user equipments UE1 and UE2respectively are co-scheduled on the same bandwidth e.g. let UE 1=firstuser equipment 105 and UE2=second user equipment 107. Further, as anexample, consider that UE1 and UE2 belong to different cells (e.g. seeFIG. 1a ) and are not assigned the same base sequence. An example isthat UE1 belongs to a macro-cell and UE2 to a pico-cell in a hetnet LTEscenario. Since the base sequences associated respectively to subframeS1 for UE1 and subframe S2 for UE2 are different, orthogonality betweenthe UEs is not possible with consequent performance degradationoccurring due to cell-edge interference.

Co-scheduling user equipments on the same time-frequency resource blockis a technique that is used to make more efficient use of the availableresources in a communications network. The embodiments herein may beapplied to an arbitrary number of co-scheduled user equipments from anyLTE releases although for simplicity the above example involves the twoco-scheduled user equipments UE1 and UE2.

The above problem is solved with the herein described embodiments byoptionally switching the base sequence employed by certain UEs in a cellfrom the cell-specific base sequence set per default to a differentbase-sequence, i.e. an alternative base sequence, which is a UE specificbase sequence. The alternative base sequence is configured per UEdependent on interference situation. That is, if one or more UEs of acell is(are) interfered by a neighbour cell, or would possibly interfereone another in the same cell if given different base sequences in caseof an MU-MIMO scenario, then these UEs of the cell are assigned analternative base sequence, which thus becomes “UE specific” as not beingdependent on the cell specific parameters of the serving cell, which isthe case for the default cell specific base sequence.

In one typical embodiment configuration, the alternative base sequencematches the default base sequence for a neighbouring cell (including thebase sequence randomization of SGH and CS hopping). For the abovedescribed example, the alternative base sequence for UE1 would then bethe default base sequence used in the neighbour cell i.e. the defaultbase sequence used by UE2.

The embodiments herein introduce using an alternative base sequence,which may be dynamically triggered or activated by way of schedulinggrants. An index of the alternative base sequence may be indicated orconfigured in a semi-static way, e.g. by RRC higher layer signaling.This provides that the signaling overhead required for dynamicallyindicating the selected base sequence is minimized and flexibility inscheduling allocation is preserved.

Whenever a switching to an alternative base sequence is triggered, allthe base-sequence specific parameters, e.g., hopping offsets for SGH andCS hopping, are correspondingly dynamically adjusted.

By choosing the alternative sequence properly, e.g., for a CoMP setting,it is possible to allow perfect or at least significantly improved RSorthogonality of Rel-11 UEs with either Rel-8/9/10/11 UE. Orthogonalityis achieved both when SGH is enabled and disabled for the Rel-8/9/10 UE.

Even though FIG. 1 show only one cell 101 it is obvious for a personskilled in the art that communications network 100 could also comprise aplurality of cells 101. FIG. 1a illustrates an example where thecommunications network 100 comprises two cells 101 each served by arespective network node 103. A communications network 100 comprising aplurality of cells 101 could be organized in various ways as is wellknown in the art. The communications network 100 could, for example, beorganized as a heterogeneous network or so-called hetnet with a macrocell comprising one or more pico cells. It could alternatively beorganized as a homogenous network with two or more macro cells served byone or more base stations so-called macro deployment or be organized tocope with a so-called hotspot scenario where an access point serves asmall area with high throughput need.

The network node 103 comprises information about a default base sequenceemployed by a user equipment 105 served by the cell. In someembodiments, the network node 103 comprises information about aplurality of neighbour cell default base sequences, each default basesequence being employed by respective user equipment(s) located in arespective neighbour cell. The information about the default basesequence(s) may be stored in the node 103 in the form of the actual basesequence(s), or as respective index's pointing to a table comprising thedefault base sequence(s). The respective default base sequence for acell 101 is common to the cell 101, and set or configured per default ineach user equipment being served by the cell 101. See for example 3GPPTechnical Specification TS36.211 section 5.5 for an example of a basesequence.

FIG. 4 in a combined signaling diagram and flow chart illustratesembodiments of a method for handling base sequences in a communicationsnetwork 100 comprising a network node 103 and one or more sets of UEs105, 107, 109 in accordance with the following method steps 401-408. Thecommunications network 100 may then comprise one or more network nodes103 serving one or more cells 101 being neighbour cell(s) to each other,i.e. being arranged proximate to or even partly overlapping each other,where the respective sets of UEs 105, 107 and 109 may belong to the samecell 101 (shown in FIG. 1), partly same cell (FIG. 1a ), or therespective sets of UEs 105, 107 and 109 may each belong to a respectivedifferent cell (not shown):

Step 401

The network node 103 receives a signal from a set of the user equipments105, 107, 109 comprised in the communications network 100 at some of thereception points belonging to the network 100. The network node 103 maybe a central scheduling unit CSU which is a scheduler controlling atleast some aspects of radio resource management (RRM) for a group ofcoordinated cells, i.e. CoMP cluster, in the network 100. In someembodiments, the network node 103 is a base station comprising thecentral scheduling unit. In the following description, the term networknode 103 will be used. The set of user equipments 105, 107, 109 maycomprise one or more user equipments distributed in one or more cells101 served by one or more network nodes 103.

Step 402

The network node 103 measures one or more properties of the signalreceived from one or more of the user equipments 105, 107, 109 withinthe network 100 at one or more reception points belonging to the network100. The reception points may be associated to all or a subset of thenodes cooperating for UL CoMP. The measured properties may be e.g., DMRSpower and/or sounding reference signal SRS power and/or position of theuser equipments 105, 107, 109 in the cell 101.

Step 403

The network node 103 identifies a first subset of user equipment(s) 105which is affected by strong interference. The identification ofinterference affected UEs 105 may be based on the measurements performedin step 402, and related to a reference signal comprised in the signalreceived in step 401. The first subset of user equipment(s) 105 maycomprise one or more user equipments.

Step 404

The network node 103 identifies a second subset of user equipment(s) 107that generate the most severe interference to the first subset of userequipment(s) 105 identified in step 403. The second subset of userequipments may comprise one or more user equipments. To identify thestrongest or most severe interference, the network node 103 may comparethe magnitude, amount or size of the measured interference with athreshold. The user equipment(s) generating interference which is abovethe threshold may be identified as the user equipment(s) among thesecond subset of user equipment(s) 107 generating the most severeinterference.

Step 405

The network node 103 determines that the first subset of userequipments(s) 105 affected by the interference should be assigned analternative base sequence. In other words, the use equipment(s) 105affected by the interference should switch from its default basesequence to an alternative base sequence, or replace the default basesequence with an alternative base sequence. Further, SGH may also beenabled, e.g. by RRC signaling, for some or all of the user equipment(s)105 in the first subset according to procedures described in 3GPPTechnical Specification TS 36.211 sections 5.5 via a pseudo-randomgenerator. In some embodiments, the alternative base sequencecorresponds to a second base sequence, such as a second default basesequence, employed in a neighboring cell comprising the second subset ofuser equipment(s) 107 which is identified as interfering or which isexpected to generate interference. The interference expectation could bebased on previous interference measurements and/or previous positioningmeasurements done i.e. based on historical data.

The network node 103 comprises information about the alternative basesequence(s) e.g. the interfering neigbour cell default base sequence,and configures the alternative base sequence(s) in the user equipment(s)105 e.g. via Radio Resource Control (RRC) signaling. In someembodiments, the network node 103 comprises information about aplurality of alternative base sequences such as base sequence(s) used asdefault base sequence(s) in some or all of the neighboring, i.e.surrounding, cells. The information about the alternative base sequencemay be the actual base sequence, or an index of the base sequence, theindex pointing to the alternative base sequence in a table comprisingthe alternative base sequence and possibly more alternative basesequences. The information of the one or more alternative basesequences, may be pre-stored in the network node 103 received via X2signaling or via signaling over proprietary interfaces from neighbourcells exchanging information of their respective employed basesequence(s), which most often, but not necessarily always, is thedefault sequence of the respective neighbour cell. The information ofthe neighbour cell default base sequences, may be proprietaryinformation pre-stored in the network node and/or may be shared betweennodes over a standardized interface. For a CoMP scenario, this info iscarried by the backhaul between coordinated CoMP nodes. A plurality ofalternative base sequences may thus be stored and indexed in a table ofthe network node 103.

In some embodiments, the alternative base sequence a user equipmentspecific base sequence, and the default base sequence is a cell specificbase sequence. A user equipment specific base sequence is a basesequence which is specific for that user equipment not only using cellspecific parameters. A cell specific base sequence is a base sequencecommon for the cell 101 being dependent on cell specific parameter andused per default for all user equipments located in the cell 101.

As mentioned above, a reference signal, such as a DMRS and a SRS, ischaracterized by a group-index and a sequence-index. The referencesignal is derived from a base sequence. Cyclic shift CS may be used forderiving the reference signal from the base sequence.

In prior art the base sequence used for DMRS is derived from cellspecific parameters. The embodiments disclosed herein is directed atmaking at least some of those parameters UE-specific. CS and OCC areapplied to the base sequences to derive the reference signals. Theembodiments herein describe selecting and using an “alternative basesequence which includes using an alternative set of initializationparameters for deriving the base sequence.

If the network node 103 comprises information about a plurality ofalternative base sequences for a plurality of user equipments 105, 107,109, the network node 103 selects the alternative base sequencespecified for the user equipment 105 or selects the index for thealternative base sequence from a set of predefined alternative basesequences made common to both the user equipment 105 and network node103 through RRC signaling.

If multiple alternative base sequences are configured in the UE 105 bythe network node 103, the node 103 may signal to the UE which one to useby signaling the index for the alternative base sequence from a set ofpredefined base sequences. Such index may be signaled by RRC or byscheduling grants (dynamic assignment).

In a CoMP scenario when two interfering user equipments from differentinterfering cells are co-scheduled on the same bandwidth, the basesequence of one of them is switched so that the alternative sequence,corresponding to the base sequence of the other user equipment, isemployed. For example, when the network node 103 has receivedinformation of the base sequence used in the neighbour cell and therebyused also by the interfering user equipment 107 of that cell, thenetwork node 103 assigns this base sequence to the interfered userequipment 105 of its cell. Thereby, the same base sequence is used forthe respective user equipments 105 and 107 in the interfering cells.

Neighbour cell base sequence information may for example be exchangedbetween the respective neighbour network nodes 103 via the X2 interfaceor by a proprietary interface.

Thus, when co-scheduling potentially highly interfering user equipments105, 107 as identified at step 404 the network node 103, e.g. thecentral scheduling unit CSU of the network node 103, assigns thealternative base sequence so that the co-scheduled user equipmentsemploy the same base sequence.

Orthogonality between the co-scheduled user equipments may then beachieved by applying CS and/or OCC.

If a user equipment ceases to generate and/or receive strong inter-cellinterference due to changed traffic conditions and/or movement withinthe cell 101, its alternative base-sequence(s) may be re-configured,i.e. the alternative base sequence currently used may be updated withanother alternative base sequence or with the initial default basesequence.

Step 406

The network node 103 assigns or configures by e.g. RRC signaling, tosome or all of the user equipments 105 in the first subset, thealternative base sequence(s) corresponding in some embodiments to thebase sequence used by the interfering second subset of user equipments107. In other words, the network node 103 configures the userequipment(s) 105 with the selected alternative base sequence(s). Thealternative sequence(s) is/are, as already mentioned, configured in theUEs 105 in a semi-static way, e.g., by Radio Resource Control (RRC)higher-layer signaling.

An indication to perform switching from the default base sequence to thealternative sequence, for a given user equipment, is either dynamicallysignaled as part of the scheduling information or it could also besemi-statically signaled via RRC signaling, not to be mistaken with theRRC configuring of the alternative sequence in the UE, which is aseparate action and could be done in advance. However the RRCconfiguring and RRC indicated switching of base sequence could also beperformed at the same time e.g. upon detecting an interference situationthereby saving signaling and time. The dynamically indicated switchingprovides the advantage that the signaling overhead required fordynamically indicating the selected base sequence is minimized andflexibility in scheduling allocation is preserved.

In some embodiments, the scheduling information might include an indexfield pointing to one of a subset of base sequences which have beensemi-statically configured in the UE 105.

In case no alternative subcarriers are configured for a given userequipment, a base-sequence switch trigger field included in thescheduling grant may be dynamically removed or de-configured, in orderto save signaling overhead. The UE then interprets DCI format for thebase sequence to be used and the switch trigger field or so-calleddynamic switching function is activated and/or deactivated by thenetwork through RRC signalling when suitable, i.e. dependent on whetheror not there is a need for replacing the base sequence as discussedabove. The need may arise for a user equipment when e.g. moving near acell edge due to enhanced risk of neighbour cell interference, i.e. whentraffic conditions or position change.

Furthermore, when one or some alternative base-sequences are configuredfor a given user equipment, the switching to one out of these sequencesthereof may in some embodiments be dynamically triggered by certain codepoints in form of data bits included in the scheduling grantcorresponding to specific CS/OCC bit combinations for the DMRS.Considering that only a subset of the user equipments, and typicallyonly cell-edge user equipments, are expected to be configured with oneor more alternative base sequences, such restriction in CS/OCCassignment flexibility is acceptable.

Step 407

The user equipment 105 switches from its default base sequence to thealternative base sequence.

The embodiments herein switch the base-sequence employed by certain userequipments in a cell from the cell-specific base-sequence, e.g. default,to a UE specific base-sequence, e.g. alternative sequence. Thealternative sequence index might be configured per-cell or per userequipment.

In case other DMRS parameters depend on the base sequence, e.g., thepseudo-random sequence for CS hopping and SGH, such parameters are alsoadjusted according to the dynamically indicated sequence.

The network node 103 may receive reference signals according to theselected alternative base sequence. Even though the configuration and/orscheduling of the alternative base sequence may be performed at onenetwork node, such as at network node 103 the reception may be performedat some other nodes, e.g. in case of a CoMP scenario.

Step 408

The network node 103 performs channel estimation of the channel betweenthe network node 103 and the user equipment 105 based on the alternativebase sequence.

The above steps from 401 are repeated, e.g., in case new user equipmentsenter/exit the coordinate cells, such as in a CoMP scenario, and/ormeasurements in step 402 are not sufficiently updated. The steps from401 may be repeated periodically.

FIG. 5 is a flowchart describing embodiments of a method in the networknode 103 for handling base sequences in a communications network 100. Asmentioned above the network node 103 is configured to communicate with afirst set of one or more user equipment(s) 105. In some embodiments, thenetwork node 103 is configured to communicate with the first userequipment 105 over a radio channel 102. The network node 103 comprisesinformation about a default base sequence and about one or morealternative base sequence(s). In some embodiments, the information aboutthe determined alternative base sequence comprises the determinedalternative base sequence or an index pointing to a table comprising thealternative base sequence. The table may be stored in a computerreadable memory in the network node 103. In some embodiments, thenetwork node 103 serves a cell 101. The network node 103 may beconfigured to communicate with the first user equipment 105 located inthe cell 101. In some embodiments, the alternative base sequence is auser equipment specific base sequence, and the default base sequence isa cell specific base sequence.

Embodiments of the method comprise steps to be performed by the networknode 103:

Step 501

This step corresponds to steps 401 and 402 in FIG. 4.

In some embodiments, the network node 103 evaluates a signal, such as areference signal, received from the first user equipment 105.

Step 501 a

This step corresponds to step 402 in FIG. 4, and is a sub step of step501.

In some embodiments, the network node 103 measures a power associatedwith a reference signal, such as SRS or DMRS, comprised in the signal.The power is measured using any suitable measuring technique for powermeasurements, for example techniques known in the art for measuringreference signal received power RSRP may be used.

Step 501 c

This step corresponds to step 402 in FIG. 4, and is a sub step of step501. Step 501 c is performed after step 501 a, or instead of step 501 aor instead of step 501 a.

In some embodiments, the network node 103 obtains a position of thefirst user equipment 105 in the communications network 100. The networknode 103 may obtain the position by using any suitable positioningmeasurement techniques known in the art, it may receive the positionfrom another node(s) in the network 100, e.g. the user equipment 105itself, or it may calculate the position using predetermined informationabout position or using information received from another node(s) in thenetwork 100.

Step 501 d

This step corresponds to step 402 in FIG. 4, and is a sub step of step501. Step 501 d is performed after step 501 a, or after step 501 c, orinstead of step 501 a, instead of step 501 c or instead of step 501 aand step 501 c.

In some embodiments, the network node 103 determines that the powerassociated with a reference signal, such as a demodulation referencesignal DMRS or a sounding reference signal SRS, and/or the userequipment position is below or within a respective threshold. This maybe done by comparing the power associated with the reference signaland/or by comparing the position of the user equipment with therespective threshold. The respective threshold may be of any suitablesize and may comprise any of a RS power metric, a distance to cell edgeor geographic boundary metric.

Step 502

This step corresponds to step 403 and step 404 in FIG. 4.

In some embodiments, based on the evaluated signal, the network node 103identifies that the first user equipment 105 experiences interference inthe communications network 100.

Step 503

This step corresponds to step 403 and 404 in FIG. 4.

In some embodiments, based on the evaluated signal, the network node 103identifies a second user equipment 107 generating the interferenceexperienced by the first user equipment 105. The second user equipment107 employs a second base sequence, and the alternative base sequencecorresponds to the second base sequence.

Step 504

This step corresponds to steps 403 and 404 in FIG. 4.

In some embodiments, based on the evaluated signal, the network node 103identifies a third user equipment 109 generating the interferenceexperienced by the first user equipment 105.

Step 505

This step corresponds to step 405 in FIG. 4.

The network node 103 determines, for the first user equipment 105, thatthe alternative base sequence should replace the default base sequence.

In some embodiments, the step of determining that the alternative basesequence should replace the default base sequence is based oninformation about interference experienced by the user equipment 105.

In some embodiments, the step of determining that the alternative basesequence should replace the default base sequence is based on theinterference identified in step 502.

Step 506

This step corresponds to step 406 in FIG. 4.

The network node 103 sends information about the determined alternativebase sequence to the first user equipment 105.

In some embodiments, the information about the alternative base sequenceis sent to the first user equipment 105 via scheduling information.

Step 507

This step corresponds to step 406 in FIG. 4.

In some embodiments, the network node 103 sends information to the thirduser equipment 109 about the determined alternative base sequence, whichalternative base sequence corresponds to the second base sequence.

Step 508

In some embodiments, the network node 103 co-schedules the first userequipment 105 with the third user equipment 109. In case the first andthird user equipment 105 and 109 are located in the same cell 101, thiscould e.g. be for a MU-MIMO scenario.

Step 509

In some embodiments, the network node 103 receives, from the userequipment 105, a reference signal according to the alternative basesequence.

Step 510

In some embodiments, the network node 103 estimates the radio channel102 between the network node 103 and the first user equipment 105 basedon the alternative specific base sequence.

To perform method steps of embodiments shown in FIGS. 5 and 9 forhandling base sequences in a communications network 100, the networknode 103 comprises an arrangement as shown in FIG. 6. The network node103 is configured to communicate with a first user equipment 105. Insome embodiments, the network node 103 is configured to communicate withthe first user equipment 105 over a radio channel 102. As mentionedabove, the network node 103 comprises information about a default basesequence and an alternative base sequence. In some embodiments, thenetwork node 103 serves a cell 101. The network node 103 is configuredto communicate with the first user equipment 105 in the cell 101. Insome embodiments, the alternative base sequence is a user equipmentspecific base sequence, and the default base sequence is a cell specificbase sequence.

The network node 103 comprises a determining unit 601 configured todetermine, for the first user equipment 105, that the alternative basesequence should replace the default base sequence. In some embodiments,the determining unit 601 is further configured to determine that thealternative base sequence should replace the default base sequence basedon information about interference experienced by the user equipment 105.In some embodiments, the determining unit 601 is further configured todetermine that the alternative base sequence should replace the defaultbase sequence based on the identified interference.

The network node 103 further comprises a sending unit 602 configured tosend information about the determined alternative base sequence to thefirst user equipment 105. In some embodiments, the sending unit 602 isconfigured to send the information about the alternative base sequenceto the first user equipment 105 via scheduling information. In someembodiments, the sending unit 602 is configured to send information tothe third user equipment 109 about the determined alternative basesequence. The alternative base sequence corresponds to the second basesequence. In some embodiments, the information about the determinedalternative base sequence comprises the determined alternative basesequence or an index pointing to a table comprising the alternative basesequence.

In some embodiments, the network node 103 further comprises a processingunit 605. In some embodiments, the processing unit 605 is configured toevaluate a signal received from the first user equipment 105. Theprocessing unit 605 may be further configured to, based on the evaluatedsignal, identify that the first user equipment 105 experiencesinterference in the communications network 100. In some embodiments, theprocessing unit 605 is further configured to, based on the evaluatedsignal, identify a second user equipment 107 generating the interferenceexperienced by the first user equipment 105. The second user equipment107 may employ a second base sequence, and the alternative base sequencemay correspond to the second base sequence.

In some embodiments, the processing unit 605 is further configured tomeasure a power associated with a reference signal comprised in thesignal, and/or measure a power associated with a sounding referencesignal, referred to as SRS, comprised in the signal; and/or obtain aposition of the first user equipment 105 in the communications network100. The processing unit 605 may be further configured to determine thatthe power associated with a reference signal, such as SRS or DMRS,and/or that the position is below a threshold.

In some embodiments, the processing unit 605 is configured to, based onthe evaluated signal, identify a third user equipment 109 generating theinterference experienced by the first user equipment 105. The processingunit 605 may be configured to co-schedule the first user equipment 105with the third user equipment 109.

In some embodiments, the processing unit 605 is configured to estimatethe radio channel 101 between the network node 103 and the first userequipment 105 based on the alternative specific base sequence.

In some embodiments, the network node 103 comprises a receiving unit 607configured to receive, from the user equipment 105, a reference signalaccording to the alternative base sequence.

The method described above will now be described seen from theperspective of the user equipment 105. FIG. 7 is a flowchart describingembodiments of the method in the user equipment 105, for handling basesequences in a communications network 100. As mentioned above the firstuser equipment 105 is configured to communicate with a network node 103.The first user equipment 105 employs a default base sequence. In someembodiments, the user equipment 105 is located in a cell 101. The cell101 is served by the network node 103. As mentioned above, in someembodiments, the alternative base sequence is a user equipment specificbase sequence, and the default base sequence is a cell specific basesequence. In some embodiments, information about one or more alternativebase sequence(s) is (are) dynamically or semi-statically received fromthe network node 103. The method comprises the steps to be performed bythe user equipment 105:

Step 701

This step corresponds to step 401 in FIG. 4.

In some embodiments, the user equipment 105 sends a signal to thenetwork node 103.

Step 702

This step corresponds to step 406 in FIG. 4.

The user equipment 105 receives, from the network node 103, informationthat an alternative base sequence should replace the default basesequence. By replacing, it is meant that the user equipment shouldemploy the alternative base sequence instead of the default basesequence. In some embodiments, replacing means that the alternative basesequence overrides the default base sequence.

Step 703

The user equipment 105 replaces the default sequence with thealternative base sequence. Thus, the user equipment 105 now employs thealternative base sequence instead of the default base sequence.

To perform method steps of embodiments shown in FIGS. 7 and 10 forhandling base sequences in a communications network 100, the userequipment 105 comprises an arrangement as shown in FIG. 8. The userequipment 105 is configured to communicate with a network node 103. Theuser equipment 105 employs a default base sequence. In some embodiments,the user equipment 105 is located in a cell 101. The cell 101 is servedby the network node 103.

The user equipment 105 comprises a receiving unit 801 configured toreceive, from the network node 103, information that an alternative basesequence should replace the default base sequence. In some embodiments,the information about alternative base sequence is dynamically receivedfrom the network node 103.

In some embodiments, the alternative base sequence is a user equipmentspecific base sequence, and the default base sequence is a cell specificbase sequence.

The user equipment 105 further comprises a processing unit 805configured to replace the default sequence with the alternative basesequence.

In some embodiments, the user equipment 105 comprises a sending unit 807configured to send a signal to the network node 103.

FIG. 9 is a flowchart describing further embodiments of a method in thenetwork node 103 for handling base sequences in a communications network100. As mentioned above the network node 103 is configured tocommunicate with a set of one or more first user equipment 105(s). Thenetwork node 103 serves a cell 101 and is configured to communicate withthe first user equipment 105 located in the cell 101 over a radiochannel 102. The network node 103 comprises information about a defaultbase sequence and about one or more alternative base sequence(s). Insome embodiments, the information about the alternative base sequencecomprises the alternative base sequence or an index pointing to a tablecomprising the alternative base sequence. The table may be stored in acomputer readable memory in the network node 103. In some embodiments,the alternative base sequence is a a user equipment specific basesequence, and the default base sequence is a cell specific basesequence. In some embodiments the information about one or morealternative base sequences have been received signalled from one or moreneighbour cells, e.g. over an X2 interface or proprietary interface. Thenetwork node 103 may further in some embodiments configure the firstuser equipment 105 with the one or more alternative base sequences viahigher layer signalling, for example via radio resource control, (RRC)signalling. In some embodiments, the network node 103 evaluates a signalreceived from the first user equipment 105. The configuring ofalternative base sequences in the first UE 105 may in some embodimentsbe initiated by the network node 103 upon detecting an actualinterference or interference potential for the first UE 105 whenevaluating the received signal or the configuring may be done perdefault for each user equipment when entering the serving cell.

The method embodiments comprise steps 901 and 902 to be performed by thenetwork node 103:

Step 901

The network node 103 determines to replace the default base sequencewhich is cell specific with the alternative base sequence.

In some embodiments the alternative base sequence is UE specific suchthat only select one or more UEs 105 in the cell 101, but not all UEs inthe cell, is configured with the alternative base sequence whilst theremaining UEs of the cell still employ the default base sequence. Evenif more than one UE of the cell employs the same alternative sequencethis sequence is still UE specific since it is not dependent on cellspecific parameters of the serving cell, which is the case for the cellspecific default sequence.

The determining may in some embodiments be based on that the first userequipment 105 experiences interference in the communications network100.

The determining may in certain embodiments be based on that the firstuser equipment 105 is determined to have a likelihood or potential ofexperiencing interference in the communications network 100. Thispotential or likelihood may according to some embodiments be determinedby establishing the position of the UE 105 in relation to a neighbourcell comprising interfering or possibly interfering UEs, such as thesecond and/or third sets of UEs 107, 109. Historical interference datamay also or alternatively be used by the network node 103 to establishinterference likelihood or probability based on knowledge of theposition of the first UE 105, i.e. based on the interference historystatistics for previous UEs of a position or area.

Step 902

The network 103 sends information to the first UE 105 that thealternative base sequence should replace the default base sequence.

The sending of information may in some embodiments comprise includingthe information in a scheduling transmission, such as in a schedulinggrant signaling, or signaling the information via RRC. The actualalternative base sequence or the index of the base sequence may then beincluded in the scheduling transmission or in the RRC transmission. Theindex is pointing to the alternative sequence in a table of one or morealternative base sequences having been previously configured in thefirst UE 105 by the network node 103.

In some embodiments, a switch trigger field is included in thescheduling information being signaled or transmitted, for triggeringi.e. activating the switching or replacing of the default base sequencewith the alternative sequence. The switching may then be dynamicallytriggered by including certain code points in the form of data bits inthis field of the scheduling grant corresponding to specific CS/OCCcombinations for the DMRS.

FIG. 10 is a flowchart describing further embodiments of a method in theuser equipment 105, for handling base sequences in a communicationsnetwork 100. As mentioned above the first user equipment 105 isconfigured to communicate with a network node 103. The first userequipment 105 employs a default base sequence and is located in a cell101 being served by the network node 103. In some embodiments, the firstUE 105 is configured with one or more alternative base sequences viahigher layer signalling, for example via radio resource control, (RRC)signalling. The configuring of alternative base sequences in the firstUE 105 may then in some embodiments be dynamically initiated by thenetwork node 103 upon detecting an actual interference or interferencepotential for the first UE 105 when evaluating the received signal. Insome embodiments, a signal is sent to the network node 103 forevaluation.

The embodiments of the method comprise the steps 1001 and 1002 to beperformed by the user equipment 105:

Step 1001

The user equipment 105 receives, from the network node 103, informationthat one of an alternative base sequence should replace the default basesequence. By replacing, it is meant that the user equipment shouldemploy the alternative base sequence instead of the default basesequence. In some embodiments, replacing means that the alternative basesequence overrides the default base sequence.

The information to replace the default base sequence with thealternative base sequence is in some embodiments signaled in ascheduling grant or via RRC from the network node 103.

Step 1002

The user equipment 105 replaces the default sequence with thealternative base sequence. Thus, the user equipment 105 employs thealternative base sequence instead of the default base sequence.

The present mechanism for handling base sequences in a communicationsnetwork 100 may be implemented through one or more processors, such asthe processing unit 805 in the user equipment 105 depicted in FIG. 8 andthe processing unit 605 in the network node 103 depicted in FIG. 6,together with computer program code for performing the functions of theembodiments herein. The processor may be for example a Digital SignalProcessor (DSP), Application Specific Integrated Circuit (ASIC)processor, Field-programmable gate array (FPGA) processor or microprocessor. The program code mentioned above may also be provided as acomputer program product, for instance in the form of a data carriercarrying computer program code for performing the embodiments hereinwhen being loaded into the user equipment 105 and/or network node 103.One such carrier may be in the form of a CD ROM disc. It is howeverfeasible with other data carriers such as a memory stick. The computerprogram code may furthermore be provided as pure program code on aserver and downloaded to the user equipment 105 and/or network node 103remotely.

Note that although terminology from 3GPP LTE-Advanced has been used inthis disclosure to exemplify the embodiments herein, this should not beseen as limiting the scope of the embodiments herein to only theaforementioned system. Other wireless systems, comprising WCDMA,Worldwide Interoperability for Microwave Access (WiMax), Ultra MobileBroadband (UMB) and GSM, may also benefit from exploiting the ideascovered within this disclosure.

Also note that terminology such as base station and UE should beconsidering non-limiting and does in particular not imply a certainhierarchical relation between the two; in general “base station” couldbe considered as device 1 and “UE” device 2, and these two devicescommunicate with each other over some radio channel.

The embodiments herein are not limited to the above described preferredembodiments. Various alternatives, modifications and equivalents may beused. Therefore, the above embodiments should not be taken as limitingthe scope of the embodiments, which is defined by the appending claims.

It should be emphasized that the term “comprises/comprising” when usedin this specification is taken to specify the presence of statedfeatures, integers, steps or components, but does not preclude thepresence or addition of one or more other features, integers, steps,components or groups thereof. It should also be noted that the words “a”or “an” preceding an element do not exclude the presence of a pluralityof such elements.

It should also be emphasized that the steps of the methods defined inthe appended claims may, without departing from the embodiments herein,be performed in another order than the order in which they appear in theclaims.

The invention claimed is:
 1. A user equipment for handling basesequences in a communications network, the user equipment beingconfigured to communicate with a network node, the user equipmentcomprising: one or more processors configured to cause the userequipment to: receive, from the network node, information that analternative UE-specific base sequence should replace a cell-specificdefault base sequence; wherein the information indicating the determinedalternative user equipment specific base sequence is separate toinformation relating to Sequence and Group Hopping, and replace thedefault cell-specific sequence with the alternative UE-specific basesequence, and transmit a reference signal derived from the alternativeUE-specific base sequence.
 2. The user equipment of claim 1, where theone or more processors are configured to cause the user equipment to:transmit a first reference signal derived from the default cell-specificbase sequence; and transmit a second reference signal derived from thealternative UE-specific base sequence.
 3. The user equipment of claim 1,wherein the default cell-specific base sequence is a base sequence thatis dependent on cell specific parameters of a cell which is serving theuser equipment and wherein the alternative UE-specific base sequence isa base sequence that is not dependent on cell specific parameters of thecell which is serving the user equipment.
 4. The user equipment of claim1, wherein the information about the alternative UE-specific basesequence is based on interference experienced by the user equipment orbased on a probability of the user equipment experiencing interference.5. The user equipment of claim 1, wherein the information about thealternative UE-specific base sequence comprises receiving thealternative UE-specific base sequence in a radio resource control (RRC)configuration being signaled from the network node.
 6. A method in auser equipment for handling base sequences in a communication network,the user equipment being configured to communicate with a network node,the method comprising: receiving, from the network node, informationthat an alternative UE-specific base sequence should replace acell-specific default base sequence; wherein the information indicatingthe determined alternative user equipment specific base sequence isseparate to information relating to Sequence and Group Hopping, andreplacing the default cell-specific sequence with the alternativeUE-specific base sequence, and transmitting a reference signal derivedfrom the alternative UE-specific base sequence.
 7. The method of claim6, comprising: transmitting a first reference signal derived from thedefault cell-specific base sequence; and transmitting a second referencesignal derived from the alternative UE-specific base sequence.
 8. Themethod of claim 6, wherein the default cell-specific base sequence is abase sequence that is dependent on cell specific parameters of a cellwhich is serving the user equipment and wherein the alternativeUE-specific base sequence is a base sequence that is not dependent oncell specific parameters of the cell which is serving the userequipment.
 9. The method of claim 6, wherein the information about thealternative UE-specific base sequence is based on interferenceexperienced by the user equipment or based on a probability of the userequipment experiencing interference.
 10. A network node for handlingbase sequences in a communications network, the network node beingconfigured to communicate with a user equipment, the network nodecomprising one or more processors configured to cause the network nodeto: send information to the user equipment about a determinedalternative UE-specific base sequence to replace a default cell-specificbase sequence; wherein the information indicating the determinedalternative user equipment specific base sequence is separate toinformation relating to Sequence and Group Hopping, and receive from theuser equipment a reference signal derived from the alternativeUE-specific base sequence.
 11. The network node of claim 10, wherein thedefault cell-specific base sequence is a base sequence that is dependenton cell specific parameters of a cell which is serving the userequipment and wherein the alternative UE-specific base sequence is abase sequence that is not dependent on cell specific parameters of thecell which is serving the user equipment.
 12. The network node of claim10, wherein the information about the alternative UE-specific basesequence is based on interference experienced by the user equipment orbased on a probability of the user equipment experiencing interference.13. The network node of claim 10, wherein the information about thedetermined alternative UE-specific base sequence to the user equipmentcomprises configuring the user equipment with the alternativeUE-specific base sequence via radio resource control (RRC) signaling.14. The network node of claim 10, the network node comprising one ormore processors configured to cause the network node to: receive a firstreference signal from the user equipment which is derived from thedefault cell-specific base sequence; and receive a second referencesignal from the user equipment which is derived from the alternativeUE-specific base sequence.
 15. A method in a network node for handlingbase sequences in a communications network, the network node beingconfigured to communicate with a user equipment, the method comprising:sending information to the user equipment about a determined alternativeUE-specific base sequence to replace a default cell-specific basesequence; wherein the information indicating the determined alternativeuser equipment specific base sequence is separate to informationrelating to Sequence and Group Hopping; and receiving a reference signalderived from the default cell-specific base sequence.
 16. The method ofclaim 15, wherein the default cell-specific base sequence is a basesequence that is dependent on cell specific parameters of a cell whichis serving the user equipment and wherein the alternative UE-specificbase sequence is a base sequence that is not dependent on cell specificparameters of the cell which is serving the user equipment.
 17. Themethod of claim 15, wherein the information about the alternativeUE-specific base sequence is based on interference experienced by theuser equipment or based on a probability of the user equipmentexperiencing interference.
 18. The method of claim 15, comprising:receiving a first reference signal from the user equipment which isderived from the default cell-specific base sequence; and receiving asecond reference signal from the user equipment which is derived fromthe alternative UE-specific base sequence.